Since secondary batteries, in which active materials being able to sorb and desorb lithium ions are used, have small sizes and high energy densities, they have been used widely as a power source for portable electronic devices. Moreover, in recent years, it has been investigated to make even vehicles have the secondary batteries on-board.
A positive-electrode active material for such secondary batteries, layered oxides, such as LiCoO2 and LiMn2O4, have been made use of mainly. In these compounds, however, oxygen atoms coordinate with transition metals to form the skeleton of crystal structure, and lithium ions then come into the spaces between them, thereby making the crystal structure stable. Accordingly, in an overcharged state where the lithium ions have been pulled out more than the setup, it becomes difficult to retain the crystal structure so that the crystal structure becomes likely to collapse. On this occasion, oxygen becomes likely to generate. Consequently, the oxidative exothermic decomposition reactions of nonaqueous electrolytic solutions have been brought about, thereby resulting in declines in the safety of battery.
In recent years, it has been proposed to use lithium phosphate-based compounds, such as LiMPO4 (where “M” is a metal, such as Mn, Fe or Co), as a positive-electrode active material. Since the lithium phosphate-based compounds have PO43− polyanions that are disposed around the central metal “M” and whose hetero element is of greater electronegativity, it has been said that the thermal stability is high, compared with that of layered LiCoO2, and the like, in which the oxygen atoms coordinate directly with the transition metal.
Moreover, as polyanion-based compounds that can be used as a positive-electrode active material, lithium silicate-based compounds can also be given. As for specific examples of such lithium silicate-based compounds, Li2MnSiO4Li2CoSiO4, and so on, can be given, as disclosed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2008-218303. Moreover, it is disclosed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2009-170401 and Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-266882 to use Li2MnSiO4 along with another oxide to produce a solid solution and then use the resulting solid solution as a positive-electrode active material.
It is possible to expect that Li2MnSiO4 and Li2CoSiO4 can be used as a positive-electrode material for high-capacity batteries while securing the safety, because Li2MnSiO4 has a theoretical capacity that is 333.2 mAh/g and the theoretical capacity of Li2CoSiO4 is 325.1 mAh/g, and because not only either of their values are high but also they are less likely to release oxygen at the time of high temperatures.